Because acoustic ink printing (AIP) avoids the clogging and manufacturing problems associated with drop-on-demand, nozzle-based ink jet printing, it represents a promising direct marking technology. While more detailed descriptions of the AIP process can be found in U.S. Pat. Nos. 4,308,547, 4,697,195, and 5,028,937, essentially, bursts of focused acoustic energy eject droplets from the free surface of a liquid onto a recording medium. By controlling the ejection process as the recording medium moves relative to droplet ejection sites, a predetermined image is formed.
To be competitive with other printer types, acoustic ink printers must produce high quality images at low cost. To meet such requirements it is advantageous to fabricate print heads with a large number of individual droplet ejectors using techniques similar to those used in semiconductor fabrication. While specific AIP implementations may vary, and while additional components may be used, each droplet ejector will include an ultrasonic transducer (attached to one surface of a body), a varactor for switching the droplet ejector on and off, an acoustic lens (at the opposite side of the body), and a cavity holding ink such that the ink's free surface is near the acoustic focal area of the acoustic lens. The individual droplet ejectors are beneficially interconnected to form a matrix array such that the selection of an individual droplet ejector is possible by selection of its associated row and column.
As may be appreciated, acoustic printing is subject to a number of manufacturing variables, including the thicknesses and stresses on the ultrasonic transducers, electromagnetic reflections on the transmission lines, variations in acoustic coupling efficiencies, and variations in the components associated with each transducer. Because of manufacturing constraints, these variables cannot be controlled sufficiently to produce uniform droplets. The result is non-uniform droplets, i.e., droplets that vary in size, ejection velocity, and/or other characteristics. Non-uniform droplet size produces undesirable intensity variations in the final image, while non-uniform droplet ejection velocity produces misaligned droplets. Non-uniform droplets may degrade the final image so much that it becomes unacceptable. Therefore, a need exists for techniques that improve the droplet uniformity in acoustic ink printing.